This invention relates to OTP (One-Time Programmable) memory cells and, in particular, to a memory cell structure with improved programmability.
An OTP memory cell stores a bit of information, a “1” or a “0,” depending on whether the cell has been programmed or not. The particular correspondence between the value of a bit and whether the cell is programmed or not is arbitrarily defined. Programming is performed by the breakdown or rupture of the gate oxide of a programming element, a MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor), in the memory cell. With the breakdown of the gate oxide of the MOSFET, a conducting plug is formed through the gate oxide to form a programmed connection between the gate electrode and a source/drain region of the MOSFET and/or the body underlying the source/drains of the MOSFET. Since a cell is programmed by creating an electrical connection, such memory cells are sometimes called “anti-fuse” OTP cells.
A problem with the programming of OTP memory cells is its variability. Gate oxide breakdown can vary widely with unwelcome decreased conductivity and large variations in the conductivity of the memory cells of an array of such cells. It is highly desirable that continued improvements in the programmed connections be achieved.
The present invention addresses one way that the programmability of OTP memory cells can be improved.